Response of Quercus robur and two potential climate change winners-Quercus pubescens and Quercus ilex-To two years summer drought in a semi-controlled competition study: I-Tree water status

In 2014, significant differences between the CO and DS treatments were only visible in Q. robur. In 2015, a longer drought period in combination with extremely high temperatures in summer led to a more profound drying out of the soil under DS. Therefore, all three species showed drought responses, with ΨPD levels dropping below ‐2 MPa and LWC values dropping below 80% LWC. Sap flow rates of DS plants were also reduced compared to CO treatments. Q. robur showed the strongest response with ΨPD below −3 MPa and premature senescence. In contrast to these physiological measurements, we could not detect differences between CO and DS plants in the D05 increase and root depth for Q. ilex and Q. pubescens, only Q. robur DS displayed strong reductions in both values comparing to CO. The response for Q. robur has already been described, but the equal performance of CO and DS plants of Q. pubescens and Q. ilex contrasts previous analyses, which award evergreen oaks, like Q. ilex, a higher maximal stomatal conductance and less embolism under DS than deciduous oaks. Interestingly, both, Q. ilex and Q. pubescens, did not show a significant reduction of growth under drought (D05, rooting depth) in our lysimeter setup, despite their significant DS responses in LWC, sap flow and ΨPD.